The melt flow rate (MFR) of a polymer resin is a function of its molecular weight. In general, increasing the melt flow rate allows the resin to be processed at lower temperatures and to fill complex part geometries. Various prior art methods of increasing the melt flow rate involve melt-blending the resin in an extruder with a compound capable of generating free radicals, such as a peroxide. The weight average molecular weight of the polymer is reduced and the MFR is increased. Increasing the melt flow rate by decreasing the molecular weight of the polyolefin polymer, however, has been found in many cases to have a detrimental effect on the strength of the modified polymer. For example, decreasing the molecular weight of the polymer can significantly lower the impact resistance of the polymer. And this lowered impact resistance can make the polymer unsuitable for use in certain applications or end uses. Accordingly, when extant technologies are utilized, one must strike a compromise between increasing the melt flow rate and undesirably decreasing the impact resistance of the polymer. This compromise often means that the melt flow rate is not increased to the desired level, which requires higher processing temperatures and/or results in lower throughputs. A need therefore remains for additives and processes that can produce polymer compositions having an increased melt flow while preserving, or even improving, the impact resistance of the polymer.
Another important physical property of a polymer resin is its melt strength. Melt strength can be generally described as the resistance of the polymer melt to stretching. The melt strength of the polymer is important because it affects all extrusion processes to some degree. For example, in extruding sheet, the melt strength of the polymer influences drawdown and sag as the sheet travels from the die to the rolls. In film blowing processes, the melt strength of the polymer affects bubble stability and determines how the film can be drawn. In blow molding processes, the melt strength of the polymer affects parison sag, which must be accounted for in order to control wall thickness in the finished article. There are many factors that can affect the melt strength of a polymer, such as the molecular weight distribution and molecular branching of the polymer. And since these factors vary from polymer to polymer, the melt strength can vary widely across different polymer grades. Accordingly, those seeking to use a particular polymer in an extrusion process often spend a significant amount of resources adapting a particular extrusion process (e.g., changing process conditions or modifying or changing equipment) in order to account for the unique melt strength of the particular polymer being used. Given the importance of polymer melt strength in extrusion processes, there is a need within the industry for additives and processes that can modify (e.g. increase) the melt strength of existing polymers. Such additives could be used to change the melt strength of a polymer so that it suits a particular process design, as opposed to the current practice of changing the process design to suit the melt strength of a particular polymer.